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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_asio.reference.async_result"></a><a class="link" href="async_result.html" title="async_result">async_result</a>
</h3></div></div></div>
<p>
        <a class="indexterm" name="boost_asio.indexterm.async_result"></a>
      </p>
<p>
        An interface for customising the behaviour of an initiating function.
      </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span>
    <span class="keyword">typename</span> <span class="identifier">CompletionToken</span><span class="special">,</span>
    <span class="keyword">typename</span><span class="special">...</span> <span class="identifier">Signatures</span><span class="special">&gt;</span>
<span class="keyword">class</span> <span class="identifier">async_result</span>
</pre>
<h5>
<a name="boost_asio.reference.async_result.h0"></a>
        <span class="phrase"><a name="boost_asio.reference.async_result.types"></a></span><a class="link" href="async_result.html#boost_asio.reference.async_result.types">Types</a>
      </h5>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                <p>
                  Name
                </p>
              </th>
<th>
                <p>
                  Description
                </p>
              </th>
</tr></thead>
<tbody>
<tr>
<td>
                <p>
                  <a class="link" href="async_result/completion_handler_type.html" title="async_result::completion_handler_type"><span class="bold"><strong>completion_handler_type</strong></span></a>
                </p>
              </td>
<td>
                <p>
                  (Legacy.) The concrete completion handler type for the specific
                  signature.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <a class="link" href="async_result/return_type.html" title="async_result::return_type"><span class="bold"><strong>return_type</strong></span></a>
                </p>
              </td>
<td>
                <p>
                  (Legacy.) The return type of the initiating function.
                </p>
              </td>
</tr>
</tbody>
</table></div>
<h5>
<a name="boost_asio.reference.async_result.h1"></a>
        <span class="phrase"><a name="boost_asio.reference.async_result.member_functions"></a></span><a class="link" href="async_result.html#boost_asio.reference.async_result.member_functions">Member
        Functions</a>
      </h5>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                <p>
                  Name
                </p>
              </th>
<th>
                <p>
                  Description
                </p>
              </th>
</tr></thead>
<tbody>
<tr>
<td>
                <p>
                  <a class="link" href="async_result/async_result.html" title="async_result::async_result"><span class="bold"><strong>async_result</strong></span></a> <span class="silver">[constructor]</span>
                </p>
              </td>
<td>
                <p>
                  (Legacy.) Construct an async result from a given handler.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <a class="link" href="async_result/get.html" title="async_result::get"><span class="bold"><strong>get</strong></span></a>
                </p>
              </td>
<td>
                <p>
                  (Legacy.) Obtain the value to be returned from the initiating function.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <a class="link" href="async_result/initiate.html" title="async_result::initiate"><span class="bold"><strong>initiate</strong></span></a> <span class="silver">[static]</span>
                </p>
              </td>
<td>
                <p>
                  Initiate the asynchronous operation that will produce the result,
                  and obtain the value to be returned from the initiating function.
                </p>
              </td>
</tr>
</tbody>
</table></div>
<p>
        The <a class="link" href="async_result.html" title="async_result"><code class="computeroutput"><span class="identifier">async_result</span></code></a>
        trait is a customisation point that is used within the initiating function
        for an <a class="link" href="../overview/model/async_ops.html" title="Asynchronous Operations">asynchronous operation</a>.
        The trait combines:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            the completion signature (or signatures) that describe the arguments
            that an asynchronous operation will pass to a completion handler;
          </li>
<li class="listitem">
            the <a class="link" href="../overview/model/completion_tokens.html" title="Completion Tokens">completion
            token</a> type supplied by the caller; and
          </li>
<li class="listitem">
            the operation's internal implementation.
          </li>
</ul></div>
<p>
        Specialisations of the trait must satisfy the <a class="link" href="asynchronous_operations.html#boost_asio.reference.asynchronous_operations.completion_tokens_and_handlers">async_result
        requirements</a>, and are reponsible for determining:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            the concrete completion handler type to be called at the end of the asynchronous
            operation;
          </li>
<li class="listitem">
            the initiating function return type;
          </li>
<li class="listitem">
            how the return value of the initiating function is obtained; and
          </li>
<li class="listitem">
            how and when to launch the operation by invoking the supplied initiation
            function object.
          </li>
</ul></div>
<p>
        This template may be specialised for user-defined completion token types.
        The primary template assumes that the CompletionToken is the already a concrete
        completion handler.
      </p>
<h5>
<a name="boost_asio.reference.async_result.h2"></a>
        <span class="phrase"><a name="boost_asio.reference.async_result.remarks"></a></span><a class="link" href="async_result.html#boost_asio.reference.async_result.remarks">Remarks</a>
      </h5>
<p>
        For backwards compatibility, the primary template implements member types
        and functions that are associated with legacy forms of the <a class="link" href="async_result.html" title="async_result"><code class="computeroutput"><span class="identifier">async_result</span></code></a> trait. These are annotated
        as "Legacy" in the documentation below. User specialisations of
        this trait do not need to implement these in order to satisfy the <a class="link" href="asynchronous_operations.html#boost_asio.reference.asynchronous_operations.completion_tokens_and_handlers">async_result
        requirements</a>.
      </p>
<p>
        In general, implementers of asynchronous operations should use the async_initiate
        function rather than using the <a class="link" href="async_result.html" title="async_result"><code class="computeroutput"><span class="identifier">async_result</span></code></a> trait directly.
      </p>
<h5>
<a name="boost_asio.reference.async_result.h3"></a>
        <span class="phrase"><a name="boost_asio.reference.async_result.requirements"></a></span><a class="link" href="async_result.html#boost_asio.reference.async_result.requirements">Requirements</a>
      </h5>
<p>
        <span class="emphasis"><em>Header: </em></span><code class="literal">boost/asio/async_result.hpp</code>
      </p>
<p>
        <span class="emphasis"><em>Convenience header: </em></span><code class="literal">boost/asio.hpp</code>
      </p>
</div>
<div class="copyright-footer">Copyright © 2003-2025 Christopher M.
      Kohlhoff<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
</div>
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